In manufacturing operations in which minerals are heat treated by passing through a rotary kiln at elevated temperature, a preheater apparatus is commonly provided at the feed or input end of the rotary kiln for preheating the incoming materials by contact with the waste heated gases which are discharged from the kiln.
Where relatively fine granular materials are involved, the preheater apparatus frequently takes the form of a series of cyclone housings which provide for a cascading flow of the granular material in contact with the heated gases. Preheaters of this general type are shown, by way of example, in U.S. Pat. Nos. 3,738,794; 4,004,876; 4,022,568 and 4,105,396.
Where the minerals undergoing heat treatment are in the form of relatively coarse aggregate, a different type of preheater apparatus must be employed. One commercially available preheater which is designed for handling relatively coarse aggregate materials operates on a batchwise basis and utilizes a device which positions a static bed of the aggregate in the flow of the heated gas, with a massive plunger device being provided for periodically emptying the bed of the preheated aggregate in preparation for refilling the bed with fresh aggregate. Other types of preheater devices designed for handling solid aggregate materials are shown in U.S. Pat. Nos. 3,159,386; 3,671,027; 3,883,294; and 4,038,025.
The previously available aggregate preheaters of which applicant is aware are of relatively massive size and are quite expensive. The preheaters typically have a number of moving parts which are subject to high temperature and to temperature changes and thus generally require a considerable amount of maintenance. Additionally, the aggregate preheaters of which applicant is aware are relatively inefficient, allowing a significant amount of usable heat energy to remain in the waste gases which are discharged to the atmosphere. Because of this inefficiency and the relatively high temperature of the waste gases discharged from the preheater, it is generally necessary with the prior types of aggregate preheaters to provide some way to cool the gases after passing through the preheater and prior to filtering the gases in a baghouse. This is typically accomplished using either an auxiliary cooling apparatus or by bleeding in ambient outside air to mix with the heated gases and thus reduce the temperature of the gases. The former approach involves additional energy consumption, while the latter approach increases the load on the filtering system and thus increases the size and expense of the filter.
With the foregoing in mind, it is an object of the present invention to provide an improved apparatus and method for preheating aggregate in conjunction with a rotary kiln and which overcomes the aforementioned disadvantages and limitations of the preheater devices and methods heretofore available.
It is another object of this invention to provide an aggregate preheater apparatus and method which is highly effective in transferring heat from the waste gases to the aggregate being processed therethrough.
It is a further object of this invention to provide an aggregate preheater of simple, inexpensive, and maintenance-free construction.
A further object of this invention is to provide an aggregate preheater which is constructed so as to assist in removing dust from the aggregate to thereby reduce the load on the filtering apparatus.
Still another object of this invention is to provide an apparatus and method of the type generally described which is useful not only for preheating aggregate, but in other applications where aggregate is treated by contact with a flowing gas.
Still another object of this invention is to provide an improved apparatus and method for continuously treating a solid aggregate with a flow of gas characterized by providing highly effective contact of the aggregate with the gas.